This invention generally relates to a process for the removal and purification of ions from solutions, and in particular, to a method and apparatus of removing fluoride ions from a waste process stream using an anion exchange resin, followed by distillation, to recover the fluoride ion as an ultrapure hydrofluoric acid.
Many industrial operations utilize fluoride, often as hydrofluoric acid or as fluoride salts such as ammonium fluoride. For example, semiconductor manufacturers utilize large amounts of hydrofluoric acid, and other fluoride compounds, to process silicon wafers. When using hydrofluoric acid (HF), semiconductor manufacturers often require an ultrapure hydrofluoric acid, defined as having all metals each below 1 part per billion (ppb), with all anions each below 10 ppb, including fluorosilicate anion, and total organic contaminants less than 100 ppb, preferably below 10 ppb.
The typical semiconductor manufacturer may produce an average of 10,000 gallons per day of a mixed acidic fluoride waste. The production of such vast quantities of fluoride ion waste, however, presents significant disposal problems. Fluoride wastes are becoming subject to increasingly stringent environmental controls for treatment and disposal. Most waste water effluent permits limit fluoride wastes to less than 10 ppm. Industry must therefore greatly reduce the fluoride content of waste solutions before the solutions may be introduced into the municipal water disposal system.
The ideal solution to the fluoride waste disposal problem would involve recycling of the fluoride from the waste and purifying it for reuse in the industrial process. Unfortunately, the current methods of removing fluoride from waste streams are not adapted to recycle fluoride wastes to produce the ultrapure HF required by industry.
The best current technology for removing the fluoride is the use of calcium salt caking system. Calcium sulfate, hydroxide, or chloride is reacted with the fluoride waste to form insoluble calcium fluoride which precipitates from solution. The remainder of the waste water constituents, including ammonia and nitrate, do not react and are sent to the drain. Drawbacks to this approach include the high amounts of residual fluoride that remain in the treated effluent and the large amounts of aqueous sludge that are created. In addition, it is very difficult for the semiconductor manufacturer to recycle the fluoride after this process has been used because caked calcium fluoride also contains high amounts of silicon as silica, which is difficult to separate from fluoride. The presence of silica in raw materials has adverse effects on many semiconductor manufacturing processes. Thus, all of the fluoride from conventional fluoride caking systems is unusable by semiconductor manufacturers being unavailable for recycle or recovery.
In addition to caking systems, anion exchange resins have been used to remove and concentrate fluoride ions from waste waters. Anion exchange resins will absorb all anions present in the waste water, including both F.sup.- (fluoride ion), and fluorosilicate (SiF.sub.6.sup.2-). Anion exchange resins can be stripped of absorbed fluoride ions by using either strongly basic solutions such as sodium or potassium hydroxide, or with strongly acidic solutions such as sulfuric acid. The anion exchange resin is typically used to exhaustion, then the fluoride and other anions which have also been absorbed from the waste water are removed by washing the ion exchange resin with strongly alkaline solution or with concentrated sulfuric acid. While this process works for removing fluoride from waste waters, silicon contamination is still a problem, because fluorosilicates also absorb on the strong base resin and are washed into the acidic or alkaline stripping solution.
Moreover, conventional ion exchange followed by distillation does not eliminate the problem of silicon contamination in recycled fluoride wastes. Fluorosilicates are difficult to separate from the desired HF by means of distillation. Fluorosilicic acid tends to codistill along with hydrofluoric acid, thus increasing the difficulty of distillation to ultrapure hydrofluoric acid containing extremely low amounts of contaminating metals and silicon. Furthermore, though most impurities can be removed by ultra high efficiency distillation, fluorosilicic acid (FSA) and its decomposition product silicon tetrafluoride are extremely difficult to separate.
The method chosen to strip the ion exchange column has little effect on the silicon contamination problem. When the resin is stripped with sulfuric acid, all of the absorbed fluorosilicic acid is released into the stripping solution. Some of the fluorosilicic acid will co-distill with water and hydrogen fluoride. The rest of the fluorosilicic acid may decompose to form hydrogen fluoride and volatile silicon tetrafluoride. The silicon tetrafluoride also co-distills and contaminates the final product, either directly as SiF.sub.4 or by reacting with the distilled HF to give new fluorosilicic acid, H.sub.2 SiF.sub.6. When the resin is stripped with strongly basic solution, several reactions occur. Fluorosilicate decomposes in alkaline solution to form silica and fluoride ions, as shown:
SiF.sub.6.sup.2- +4 OH.sup.- .fwdarw.SiO.sub.2 +2H.sub.2 O+6 F.sup.-. PA1 (a) adjusting the pH of the solution to an alkaline pH to release fluoride ion from the fluorosilicic acid and the complex metal fluorides; PA1 (b) removing the fluoride ions and other anions from the solution by passing the solution through an ion exchange resin, where the ion exchange resin is adapted to adsorb substantially all of the fluoride passed over the ion exchange resin; PA1 (c) displacing the fluoride ions and other anions bound to the ion exchange resin, thereby forming a mixture of anions in an effluent emanating from the resin; PA1 (d) optionally concentrating the effluent in the form of a basic solution and then acidifying it; and PA1 (e) distilling the mixture of anions in the effluent from a sulfuric acid solution to generate the ultrapure hydrofluoric acid.
Because the stripping solution is strongly alkaline, the silica will react with the free alkali to form highly soluble silicates, SiO.sub.3.sup.2-. Thus, fluoride and silicon, in the form of silicates or silica, remain mixed together in the solution. Sulfuric acid is next added to allow distillation of HF. All of the above reactions are reversed upon acidification of the solution, and fluorosilicate is formed again. There is no net purification of the separated fluoride from the silicon contamination.
Previously disclosed methods for reducing silicon contamination are overly complicated or do not solve the current problem. German Patent 1,811,178 describes a process for neutralizing fluorosilicic acid with ammonia followed by filtration to remove silica. The ammonium fluoride is then precipitated using a metal salt, and anhydrous HF prepared using a high temperature decomposition process. U.S. Pat. Nos. 4,144,315 and 4,067,957 cover similar processes which use alkali metal precipitation of the fluoride followed by thermal decomposition.
U.S. Pat. No. 4,062,930 describes a method for destructive distillation of fluorosilicic acid from concentrated sulfuric acid, which releases gaseous silicon tetrafluoride byproduct. This does not solve the problem, however, because the silicon tetrafluoride is difficult to remove from the purified hydrofluoric acid. Much other patent literature in the manufacture of hydrofluoric acid from calcium fluoride exists which note the necessity of using low silica calcium fluoride as a raw material.
One process utilizing ion exchange resins for purification of process solutions of hydrofluoric acid is disclosed in U.S. Pat. No. 4,952,386. A weak base (weak anion) exchange resin is charged by conversion to the hydroxide form with a solution of an alkaline material such as ammonia or sodium hydroxide. The resin in rinsed to remove excess alkali. A solution of very high purity hydrofluoric acid is then passed through the weak base resin. Hydrofluoric acid in the form of unionized HF molecules is absorbed on the weakly basic amine ion exchange sites. This process continues until all of the sites are filled with HF. At this point the resin is ready for use, which is the removal of anionic contaminants from acidic hydrofluoric acid process solutions from the semiconductor industry.
A strong acid (strong cation) ion exchange resin is used to remove all positive ions from the waste stream and replace them with hydrogen ions. The hydrofluoric acid solution containing only anionic contaminants now passes through the treated weak base resin. Most anionic contaminants such as metal fluoride complexes are more tightly bound to the weak base resin than is HF. The waste stream has its contaminants removed and replaced with HF. The clean solution of HF is now ready for replenishment and reuse.
There are some drawbacks to this modified ion exchange purification procedure. This process works well to purify fluoride-containing process streams, but does not remove fluoride ion so cannot be used to concentrate or recover fluoride from waste water streams. Furthermore, there is no concentration of the purified HF solution. Consequently the purified solution is usually more dilute than the original process solution, so concentrated HF from another source must be added to regenerate the strength of the process solution. Another problem is that many of the metal fluorides are bound to the resin only slightly more efficiently than the HF. This limits both the total amount of HF which can be processed, and the purification which results. Furthermore, there is no removal of organic contaminants with this method. Finally, the process is less economical than is desired since the resin must first be charged with expensive pure HF; then the HF is lost when ammonia or caustic is used to regenerate the weak base resin. Economics are also unfavorable because of the strong acid resin which must be used prior to the weak base resin.
A further disadvantage to this process for recycling fluoride wastes results from the use of both strong acid and weak base ion exchange resins. Many fluoride containing solutions used in semiconductor manufacturing contain large amounts of ammonium fluoride, known as buffered oxide etchant (BOE). If a strong acid resin is used with BOE, all of the ammonium fluoride is converted to hydrofluoric acid. The strong acid resin must be regenerated very often at high cost. The ammonia in the BOE is lost during regeneration. Thus the above process is not useful with BOE process solutions.
Thus, there exists a need for a process and apparatus which can be used to treat all of the fluoride waste from a semiconductor manufacturing facility to separate substantially all the fluoride from the waste water, and recover it as an ultrapure concentrated hydrofluoric acid suitable for reuse.